A rare bacterial infection called tularemia that killed a Chilmark man three years ago appears to have hit the Island for the fourth summer in a row, possibly infecting as many as four people since May.

State public health officials said yesterday that they are evaluating four probable cases of tularemia, all of them either landscapers or people who work outdoors.

The disease, treatable with intravenous antibiotics, inexplicably thrives on the Vineyard. Since 2000, health officials have confirmed 22 cases of tularemia on the Island and identified landscapers as the most vulnerable to contracting what is supposed to be a rare disease.

In a typical year, there are just one or two cases of tularemia statewide.

Scientists from both state and federal agencies have investigated the outbreak but have no solid clues to why the disease remains so virulent on the Vineyard.

"Clearly, there is still a risk, and it's unusual to have an outbreak sustained for so long. Usually, it's episodic and periodic," said Dr. Fredric Cantor, the State Department of Public Health veterinarian keeping watch over tularemia and other diseases passed from animals to humans.

"It's still an issue, and people need to be aware and have a high index of suspicion," said Dr. Cantor.

The four probable cases are all men between the ages of 26 and 49. Blood tests on two of the men detected very high level of antibodies. Onset of symptoms came in mid-May for the first potential victim.

Other probable victims were likely infected earlier this month or late June, said Dr. Cantor. Results from a second round of blood tests on the four are expected by the end of the month, and will determine whether public health officials will confirm these as official tularemia cases.

One of the four cases is almost surely the pneumonic form of the disease, added Dr. Cantor. That would fit the pattern for the outbreak. All but five of the 22 confirmed tularemia cases have been pneumonic.

No other place in the U.S. has seen an outbreak of pneumonic tularemia, a disease more commonly passed on to humans from a dog tick bite.

But the fact that nearly all of the Vineyard cases have been pneumonic has bolstered the theory that landscapers are likely inhaling dirt or grass particles laced with tularemia bacteria which then infects their lungs.

Of the 22 confirmed cases, 18 were either landscapers or people who worked outdoors on a regular basis.

Epidemiologists from the federal Centers for Disease Control have flown to the Island three times - trapping rodents, collecting samples from lawn mowers and taking human blood samples - in an effort to find some evidence of why tularemia flourishes here.

Their findings were disappointing: Of the 40 animals trapped by the CDC in the summer of 2000, only two tested positive for tularemia - a Chilmark skunk and a rat from Katama. Of the more than 500 dog ticks collected from the Island, only one tested positive.

"We need to clarify what's going on with the animal population on the Vineyard," said Dr. Cantor. "There is a concern that this has been going on now for a number of years."

What the scientists call a "reservoir species" is the object of their search: finding the kind of animal that hosts the disease and passes it on.

Epidemiologists, parasitologists and microbiologists have all agreed that would be the key.

Elsewhere, a tularemia outbreak would be preceded by a massive die off of a species. But no one has seen that on the Vineyard.

Dr. Sam Telford, a Lyme Disease expert and professor at Tufts University who has continued his field studies of the Vineyard tularemia outbreak, first suspected the sizeable rat population.

But of 11 rats tested by CDC tested and another 15 dissected and put under the microscope at Mr. Telford's lab at Harvard University, there was still just one positive result.

"We need to understand how the transmission is occurring and what are the reservoirs on the Island," said Jeannine Petersen, the microbiologist in charge of tularemia at the CDC's division of vector borne infectious diseases in Fort Collins, Colo. "There are a lot of questions."

There's also no shortage of theories. Is weather a factor? The wet spring could have created a friendly breeding ground for the bacteria, said Dr. Cantor. According to a CDC study published in 2001, the bacteria "can survive in water, soil and decaying animal carcasses. The organism persists in water and mud for as long as 14 weeks, in straw for six months and in oats for four months."

As for the Island animal population, rabbits have clearly been the focus of investigations so far. They are well-known carriers, but they die from the disease. Mr. Telford has marveled at how the disease can persist when the bacteria is so deadly.

"It's a very strange parasite because it kills most of its hosts," he said. "This clever parasite doesn't."

The pneumonic form of the disease can have a mortality rate in humans as high as 60 per cent if left untreated.

The Vineyard saw another tularemia outbreak back in 1978. But of the 15 cases that year, seven of them were linked to one cottage in Chilmark where a wet dog shaking itself is thought to have aerosolized the bacteria.

Mr. Telford is convinced that both Chilmark and Katama are hot spots for tularemia.

Several of the victims are thought to have been exposed to tularemia in Katama in Edgartown and in the Squibnocket area of Chilmark. David Kurth, who died from tularemia in August 2000, had been mowing a lawn near Squibnocket about a week before he developed flulike symptoms and then failed to seek medical attention soon enough.

Public health advisories have urged landscapers to wear dust masks when they work to ward off tularemia particles. They also warned people to seek immediate medical attention if they "develop fever or respiratory symptoms within seven days of potential exposure to aerosols of dust, soil or grasses, or after direct contact with a wild animal."

Skin sores or swollen lymph glands after a tick bite are considered telling symptoms of the more common form of tularemia. For more public health information, people may call DPH at 617-983-6800.